The present invention relates to a process for the production of olefins.
Olefins such as ethylene and propylene may be produced by the catalytic dehydrogenation or cracking of a hydrocarbon feed. In this specification the term “cracking” will be used to embrace both of these chemical reactions.
The cracking of hydrocarbons is an endothermic process. Accordingly, heat has to be consumed for the reaction to occur. In a process known as auto-thermal cracking, the heat required for cracking is generated by combusting a portion of the original feed stock. This is achieved by passing a mixture of a hydrocarbon feed and an oxygen-containing gas over catalyst capable of supporting combustion beyond the fuel rich limit of flammability. The hydrocarbon feed is partially combusted, and the heat produced by the combustion reaction is used to drive the cracking of the remainder of the feed. An example of an auto-thermal cracking process is described in EP-A-0332289.
Generally, in known auto-thermal cracking processes, a reactant stream of a hydrocarbon and an oxygen-containing gas are passed over a single catalyst bed to produce product olefin. Typically, the catalyst bed comprises at least one platinum group metal, for example, platinum, supported on a catalyst support. Recently, research has been conducted on how to improve the selectivity of these catalysts to olefin product. One method is to modify the catalysts with a metal promoter from Groups IIIA, IVA, VA of the Periodic Table and/or from the transition metals. For example, WO 97/26987 discloses that the selectivity of platinum catalysts may be enhanced by incorporating tin or copper onto the supported platinum catalyst.
In WO 97/26987, the promoted platinum catalysts are prepared by impregnating a catalyst support in a platinum-containing solution, and thereafter into a solution containing the tin or copper promoter. As a result, the platinum and copper or tin promoter are distributed uniformly throughout the support. After prolonged use, however, the concentration of promoter on the support may decrease through evaporation, leading to a loss of activity.
According to Journal of Catalysis 191, 62–74 (2000), the problem of loss of promoter may be addressed by adding the tin to the catalyst by an on-line addition technique. More specifically, an aqueous solution of SnCl2 may be added to the hot operating catalyst support to deposit a thin coating of metal on to the front surface of the support. This compensates for the loss of tin due to evaporation, and restores the catalyst's initial performance. This in-situ regeneration technique is also described in Catalysis Letters 63 (1999), 113–120.
The catalytic behaviour of a supported Cr2O3 catalyst is studied in detail in Applied Catalysis A (1999), 187(1), 13–24. This reference proposes the theory that the Cr2O3 catalyst exhibits a distinctive boundary between the oxidising environment at the front of the catalyst and the reducing environment at the rear of the catalyst. In the oxidising environment near the front face of the catalyst, the catalyst acts as an oxidative dehydrogenation catalyst. Once the majority of the oxygen has been consumed at the front end of the catalyst, however, Cr2O3 acts as a dehydrogenation catalyst, using the heat generated by oxidation reactions at the front of the catalyst to crack the hydrocarbon feed. To support this premise, a Pt-coated monolith was placed in front of a series of Cr2O3 monoliths. At high C2H6/O2 ratios, the arrangement showed higher C2H4 selectivities than the Pt-monolith alone, confirming that Cr2O3 can utilise heat generated by the exothermic oxidative reactions occurring over the Pt-monolith to crack any unreacted hydrocarbon in the feed.
The teaching of Applied Catalysis A (1999), 187(1), 13–24, however, is very specific to Cr2O3. There is nothing in the reference to suggest that other catalyst beds could be employed to increase olefin selectivity in a corresponding manner.